Anthraquinone dyes have been very well known for the dyeing of synthetic fiber-based substrates. Some of these dyestuffs have found particular utility in the disperse, continuous and thermosol dyeing of polyester fabric. Among these dyes, the dye of the above formula has been of particular interest because of its superior lightfastness. Unfortunately, its utility has been limited by its instability in aqueous dispersion. In particular, on being subject to the dyeing temperatures utilized in either "at the boil" or pressurized exhaust dyeing (100.degree. to about 140.degree. C.) aqueous dispersions of this dyestuff form unacceptably large agglomerates. This, in turn, leads to deposits of precipitated dyestuff on the fiber, giving rise to stippled or specky dyeings and to incomplete exhaustion of the dye bath. Surprisingly, a modification in the crystal form of this dyestuff renders its aqueous dispersion stable under these conditions leading to level dyeings without specking and essentially complete exhaust of the bath.
Apparently, this crystal modification can occur to some extent if the known crystal form is subjected to exhaust dyeing conditions. However, this transformation is evidently accompanied by crystal growth which renders aqueous dispersions unstable. Therefore, the art did not utilize this dyestuff in such dyeing procedures in spite of its excellent lightfastness. It has now been discovered that this latter crystal form is stable and resistant to crystal growth at exhaust dyeing temperatures. Therefore, if an appropriate particle size is utilized, its aqueous dispersion remains stable.